Noritz NR50, NR66 & NR71 Code 59
Heat exchanger flow fault. Noritz puts scale first: check for scale build-up in the heat exchanger, then abnormal combustion, then the flue for blockage or obstruction, then that the flame ignites all the way across the burner. Noritz notes the code does not appear in the manual's own flowchart. On a unit with no remote controller it shows as continuous flashing, and Noritz records that the heater is available to burn even before you reset it.
Also displayed as: continuous flashing
DIY fixThe verdict
Descale the unit
Start with the flush, because Noritz does. A scaled heat exchanger restricts flow, which is what this code measures, and a flush is a kit and an afternoon against a heat exchanger that is most of the value of the appliance. The unit will keep running in the meantime, which is exactly how this gets ignored until something more expensive breaks.
Typical cost: $40–$900
US estimate for parts and labor. Prices vary by region and by who does the work — see how we estimate costs.
What the manual says
The meaning above is transcribed from the manufacturer's diagnostic documentation for the NR50, NR66 & NR71 — not averaged across the brand.
Source: Noritz Service Manual — NR50 / NR66 / NR71 / NR98 / NCC199 / NRC111 / NR111 / NC380 (Rev. July 2011) (p. 54), read Sep 2026 — the manufacturer's own documentation for this exact series.
What you can safely check
- Flush the heat exchanger — our descaling guide covers it end to end.
- Check the vent terminal outside while you are at it, since the flue is one of Noritz's causes.
- If the code survives a proper flush, book a service call rather than flushing again.
- Afterwards, work out your water hardness and set a flushing interval from it.
Frequently asked questions
The unit still heats water. Can I leave it?
You can, and Noritz's own flash table says so — this is one of the faults where the heater is available to burn even before being reset. That is the trap. A code you can ignore is a code that gets ignored, and the thing it is warning about is the heat exchanger, which is the most expensive part of the appliance.
Code 59 on other Noritz series
The same code number can mean something different on another series — make sure you're reading the page for your exact unit: